Fluid management system

ABSTRACT

Apparatus for supplying liquid to a body cavity during an endoscopic procedure includes a feedback loop-controlled liquid supply device inherently capable of supplying liquid at a substantially constant pressure substantially independent of the flow rate of liquid delivered by the liquid supply device within a relatively wide range of flow rate. Also disclosed are a disposable plastic pump cassette having an inflow pump in a housing, and an operative cannula. The surgical procedure is performed with continuous control over the body cavity pressure, regardless of the outflow flow rate.

This is a continuation of application Ser. No. 08/255,281, filed Jun. 7,1994, now abandoned, which is a continuation of Ser. No. 07/867,981,filed Apr. 13, 1992, now abandoned, which is a continuation-in-part ofSer. No. 07/748,249, filed Aug. 21, 1991, now abandoned, which is acontinuation-in-part of Ser. No. 07/838,465, filed Feb. 19, 1992, nowabandoned.

The present invention relates to apparatus for supplying liquid underpressure to a body cavity during an endoscopic procedure, and moreparticularly to an improved arthroscopic pump apparatus.

Pumping systems have been previously proposed for supplying liquid underpressure to a body cavity by means of an inflow pump during anendoscopic procedure, such as arthroscopic surgery. Two prior artarthroscopic pumping systems are illustrative, namely that of DeSatnicket al. U.S. Pat. No. 4,650,642, issued Mar. 17, 1987 and Mathies et al.U.S. Pat. No. 4,902,277, issued Feb. 20, 1990. DeSatnick et al. proposethe use of an open loop displacement pump as the inflow pump and an openloop suction pump as the outflow pump. Mathies et al. propose the use ofa feedback loop controlled inflow pump and an open loop suction pumpthat is automatically switched between an outflow cannula and a tool.

Displacement pumps operate by providing volumes of fluid as a functionof pump speed regardless of the outlet pressure of the pump. Thus, evenat low pump speeds, displacement pumps are capable of generatingpressures that can be too high, e.g., where the outflow from thepressurized body cavity is curtailed by blockages or the like. Thepresent invention provides a safer system by utilizing a liquid supplymeans whose outflow flow rate is inherently inversely responsive tochanges in the body cavity pressure. Preferably, the liquid supply meansis inherently capable of supplying liquid at a substantially constantpressure substantially independent of the flow rate of liquid deliveredby the liquid supply means within a relatively wide range of flow rate.The liquid supply means of the invention, when regulated by a feedbackloop responsive to the pressure in the body cavity, can provide precisecontrol of body cavity pressure regardless of changes in the volume ofthe body cavity or changes in the outflow flow rate from the bodycavity, such as flow through a surgical tool, leakage or blockage orotherwise.

The liquid supply means may be a reservoir of liquid that isisometrically pressurized. Preferably, the liquid supply means is adynamic pump means, such as a centrifugal pump. As used throughout thepresent application, the terms "dynamic pump" and "displacement pump"are as defined in the classification of pumps set forth in PumpHandbook, Edited by I. J. Karassik et al., Second Edition, McGraw-HillBook Company, pages 1.2-1.5, which is incorporated herein by referencethereto. A dynamic pump used in the present invention preferablyprovides a constant pressure head in a manner substantially insensitiveto variations in flow rate.

The apparatus of the present invention may include a suction pump foraspirating liquid from the body cavity. Alternatively, liquid may beaspirated from the body cavity from a suction source, such as wallsuction. According to the present invention, outflow flow rate isindependent of body cavity pressure and instead is chosen to optimizethe performance of the surgical tool being used. The combination of adynamic inflow (or irrigation) pump, such as a centrifugal inflow pump,controlled in response to body cavity pressure and an open loopaspiration of liquid from the body cavity provides a safe, stable,pressure-limited control system.

It is a particular advantage of the present invention that it ispreferably a two-portal system. That is, only two cannulas are insertedinto the surgical site, an inflow cannula for the endoscope and anoutflow cannula for the surgical tool. Liquid under pressure is normallysupplied to the body cavity by the liquid supply means through theannulus between the inflow cannula and an endoscope in the cannula,while liquid is aspirated from the surgical site by suction via theoutflow cannula or a surgical tool inserted into the outflow cannula.Alternatively, inflow liquid can be supplied via an endoscope having aninternal conduit for supplying liquid to the body cavity. While notpresently preferred, outflow liquid can simply drain from the outflowcannula. Moreover, body cavity pressure is calculated from the output ofa pressure sensor in communication with the inflowing liquid at alocation upstream of the body cavity, thus eliminating the need for athird portal for a pressure sensor inserted into the body cavity. In oneembodiment of the invention, the pressure sensor is closely adjacent thebody cavity, for example at the connection of an endoscope to the tubingsupplying inflow liquid to the body cavity via the endoscope. In apresently preferred embodiment of the invention, the pressure sensorsenses pressure at the outlet of the inflow pump located within adisposable cassette.

The present invention also provides a disposable cassette containing theinflow pump means. The cassette may also include an outflow suction pumpmeans. Preferably, the cassette will also include a resilient tubingthat may be connected in series with conduit means for aspirating liquidfrom the body cavity such that crimping of the resilient tubing meansrestricts the flow of liquid aspirated from the body cavity. Thecassette is preferably provided with a thin, flexible diaphragm inliquid communication with the outlet of the inflow pump means.

The present invention also provides an operative cannula that can serveas the outflow cannula. The cannula is provided with a sealing means atits proximal end for sealingly engaging tools passing through thesealing means proximally to distally and distally to proximally.Preferably, the operative cannula can be cut to provide a shortercannula with a tapered distal end.

The present invention is illustrated in terms of its preferredembodiments in the accompanying drawings, in which:

FIG. 1 is a schematic illustration of the apparatus of the invention;

FIG. 2 is a block diagram of a control system of the present invention;

FIG. 3 is a curve showing the relationship between the pressure dropfrom the pressure sensing means to the inflow cannula and the inflowflow rate;

FIG. 4 is a family of curves showing the relationship between the inflowpump pressure and the inflow flow rate at various pump speeds;

FIG. 5 is a curve showing the relationship between the pressure dropfrom the inflow pump to the pressure sensing means and the inflow flowrate;

FIG. 6 is a view in section of an operative cannula;

FIG. 6A is an enlarged detail view of the seal in the operative cannula;

FIG. 7 is a view in section of an irrigation extender;

FIG. 8 is a view in section showing the operative cannula and irrigationextender coupled together to form an inflow cannula assembly, with anendoscope inserted in the inflow cannula assembly;

FIG. 9 is a detail view in section of a pressure sensing means;

FIG. 10 is a view in section of a blade of a surgical tool inserted inthe operative cannula;

FIG. 11 is a top plan view of a pump cassette of the present invention;

FIG. 12 is a view in section along lines 12--12 in FIG. 11;

FIG. 13 is a top plan view of the impeller of the centrifugal pump;

FIG. 14 is a view in section taken along lines 14--14 in FIG. 11;

FIG. 15 is a top plan view of one of the motors used to drive the pumps;

FIG. 16 is a block diagram of another control system of the presentinvention;

FIG. 17 is a plan view, from the rear, of another pump cassette of theinvention;

FIG. 17A is a detail exploded view, in section, of a portion of the pumpcassette shown in FIG. 17;

FIG. 18 is a view in section taken along lines 18--18 in FIG. 17;

FIGS. 19A and 19B are views in section taken along lines 19--19 in FIG.17;

FIG. 20 is a plan view, partly in section, of an operative cannula ofthe invention;

FIG. 21 is a view in section taken along lines 21--21 in FIG. 20;

FIG. 22 is a view in section of an adaptor;

FIG. 23 is a detail view, partly in section, of a modified distalobturator tip for the operative cannula of FIG. 20; and

FIG. 24 is a plan view, partly in section, of an obturator cannula ofthe invention.

Referring to FIG. 1, the apparatus of the present invention includes afluid source 1, usually two bags of sterile saline, communicating viaconduit 2 with the inlet of a centrifugal inflow pump 4 (FIG. 2) housedwithin a cassette 200 in fluid management unit 5. Centrifugal inflowpump 4 supplies liquid under pressure through its outlet to conduit 7and thence to the body cavity 10. As will be discussed in detailhereinafter, inflow cannula assembly 9 is inserted into body cavity 10and endoscope 8 is placed within inflow cannula assembly 9 such thatliquid under pressure is delivered by centrifugal inflow pump 4 to bodycavity 10 via the annulus between the endoscope 8 and the inflow cannulaassembly 9.

Pressure transducer 11 is located upstream of the body cavity 10 andsenses the liquid pressure in conduit 7 as will be described in detailhereinafter. In the embodiment of the invention shown in FIGS. 1 and 2,pressure transducer 11 is at the connection between conduit 7 andendoscope 8. The output from pressure transducer 11 is sent via line 12to a pressure signal processor 13 (FIG. 2) within fluid management unit5 for computing the pressure within body cavity 10. The desired pressurefor the body cavity will be set by operating a selector 5a on fluidmanagement unit 5 and display 5b will display the set pressure. Pressurecontroller 40 (FIG. 2) within fluid management unit 5 is responsive tothe body cavity pressure signal generated by pressure signal processor13 to adjust the speed of centrifugal inflow pump 4 to increase ordecrease the outlet pressure of the liquid delivered by centrifugalinflow pump 4 and thereby maintain the body cavity pressure at thedesired set value, as will be explained in detail hereinafter.

An outflow cannula 14 (described in detail hereinafter) is inserted intothe body cavity 10. Tool 15 is inserted via outflow conduit 14 into thebody cavity 10, and the surgeon will manipulate tool 15 while viewingthe procedure via endoscope 8, as is conventional. Tool control unit 21powers tool 15 via power cord 22. Outflow conduit 16 connects the inletof a suction pump 18 (FIG. 2) within unit 5 with the outlet of tool 15,whereby liquid within body cavity 10 may be aspirated and sent to wastecontainer 19 via conduit 20. Fluid exiting body cavity 10 is filtered bytissue trap 16a located upstream of suction pump 18.

Endoscope 8 and tool 15 are reused after sterilization. It is presentlypreferred that the pumps 4 and 18, the pressure transducer 11, and theconduits are disposable, as well as the blades for the tool 15.

During the preoperative stage, the surgeon will select a desiredpressure for the body cavity for the given surgical procedure byoperating a set-pressure selector 5a, thereby causing set pressuregenerator 30 (FIG. 2) to generate a set pressure signal 31, which isapplied to an input of pressure controller 40. To the other input ofpressure controller 40 is applied signal 32a corresponding to thepressure in body cavity 10.

Body cavity pressure signal 32a may be calculated by pressure signalprocessor 13 using a lumped parameter model as follows. By empiricallydetermining the pressure vs. flow characteristics of each of thecomponents in the path of fluid flow, (from fluid source 1 to distal end92 (FIG. 1) of inflow assembly 9), one can determine body cavitypressure by calculating pressure drops across each component. One methodtakes into account the individual pressure vs. flow characteristics ofcentrifugal inflow pump 4, conduit 7 and inflow cannula assembly 9.While this will require more demanding software calculations, it willallow easier theoretical modeling of the effects of changing thepressure vs. flow characteristics of any component. Another method lumpsthe pressure vs. flow characteristics of the inflow pump 4 and conduit 7up to the point of pressure measurement by transducer 11. This methodsimplifies the calculation of the body cavity pressure, but it isspecific to the design of the inflow pump and conduit 7. Either methodmay be used in the present invention.

For either method, the body cavity pressure, P_(c), is calculated bysubtracting the pressure drop across the inflow cannula assembly 9 fromthe pressure, P_(s), sensed by transducer 11. The pressure drop acrossinflow cannula assembly 9, (P_(s) -P_(c)), was empirically determinedfor various flows, Q_(in), by measuring the pressure at the outlet 92 ofinflow cannula assembly 9 at a given flow, Q_(in), and subtracting itfrom the sensed pressure, P_(s), at that flow. A typical curve plotting(P_(s) -P_(c)) versus flow is schematically shown in FIG. 3, and wasaccurately fit using the function:

    (1) P.sub.s -P.sub.c =A.sub.1 Q.sub.in.sup.2 +A.sub.2 Q.sub.in

where A₁ and A₂ are constants, which can be rewritten as:

    (2) P.sub.c =P.sub.s -A.sub.1 Q.sub.in.sup.2 -A.sub.2 Q.sub.in

The values for constants A₁ and A₂ are determined and stored in memoryfor use by pressure signal processor 13.

A more generic method used to determine cavity pressure calculates theinflow, Q_(in), by solving two simultaneous nonlinear equations asfollows. The relationship between inflow, Q_(in), and pump pressure,P_(p), can be determined empirically by plotting P_(p) vs. Q_(in) forthe inlet pump 4 for a range of pump speeds, (RPM). A family of curveswas obtained for P_(p) vs. Q_(in) over a range of pump speeds (RPM), andis schematically shown in FIG. 4. For a given RPM, and using acentrifugal inflow pump connected to fluid source 1 via conduit 2 of0.25 inch ID and 90 inches long and made of PVC, the equation wasdetermined to be:

    (3) P.sub.p =B.sub.0 +B.sub.1 Q.sub.in.sup.2 +B.sub.2 Q.sub.in

which can be rewritten as: ##EQU1## using the assumption that Q_(in)must always be positive.

In this equation, B₀ is the constant obtained at zero flow, whichincludes a component derived from the height of the fluid source (e.g.the bag of saline 1), above the inflow pump 4.

As the speed of motor 4a is varied, the values for B_(n) change, but thegeneral function does not. By repeating this pressure vs. flowexperiment for many different motor speeds, functions are determined forB_(n) as functions of motor speed. The speed of motor 4a (FIG. 2) can bedetermined by employing a feedback tachometer, but the actual motorspeed is assumed to be a direct function of the pulse width modulationsignal 61 (FIG. 2) used to drive motor 4a to be described in detailhereinafter. The functions for each B_(n) can be expressed as a functionof RPM by:

    (5) B.sub.n =f(RPM)

and RPM can be expressed as a function of the pulse width modulationsignal, (PWM), by:

    (6) RPM=C..sub.PWM

Pressure transducer 11 senses the pressure of liquid in conduit 7 at apoint between centrifugal pump 4 and the inflow cannula 9. Therelationship between the inflow, Q_(in), and the pressure drop from thecentrifugal inflow pump 4 to the transducer 11 can also be determinedempirically by measuring P_(p) (pump outlet pressure) and P_(s) (sensedpressure) over a range of flow rates and fitting the resulting (P_(p)-P_(s)) vs. Q_(in) data, such as schematically shown in FIG. 5, to anequation. Using a conduit 7 of 0.25 inch ID and ten feet long made ofPVC, the function was determined to be:

    (7) P.sub.p -P.sub.s =D.sub.1 Q.sub.in.sup.2 +D.sub.2 Q.sub.in

which can be rewritten as: ##EQU2## on the assumption that Q_(in) mustalways be positive.

Given the sensed pressure, P_(s), and the pulse width modulation signal,(PWM), cavity pressure, P_(c), is calculated in the following manner:

a) Given PWM, calculate RPM from equation (6);

b) Given RPM, calculate B_(n) from equation (5);

c) Given B_(n) and P_(s), solve equations (4) and (8) for the twounknowns, Q_(in) and P_(p) ;

d) Given P_(s), A_(n) and Q_(in), calculate P_(c) from equation (2).

Pressure signal processor 13 may be provided with any suitable algorithmto solve equations (4) and (8) for unknowns P_(p) and Q_(in). Suitablealgorithms are commercially available. Equations (2), (5), and (6) aresolved using appropriate software routines of a conventional nature.

Pressure signal processor 13 then generates a signal corresponding tothe body cavity pressure, P_(c), which is sent via lines 32 and 32a tothe input of pressure controller 40 and via lines 32 and 32b to apressure display 5b on the face of fluid management unit 5 (FIG. 1).Processor 13 can generate a signal representing the flow rate, Q_(in),of liquid into the body cavity, which can be sent via line 33 to adisplay 5b on unit 5 showing the inflow flow rate. If desired, inflow,Q_(in), can be integrated over time to estimate the total volume ofliquid infused and the result displayed on unit 5 (not shown), so thatthe physician can monitor liquid usage and avoid unexpected depletion ofthe liquid supply. Since the pressure is maintained within narrowlimits, Q_(in) can be accurately measured.

Pressure controller 40 compares the set pressure signal applied via line31 and the calculated body cavity pressure signal applied via line 32aand generates a signal representing a pump speed necessary to raise orlower the calculated body cavity pressure to equal the set pressure.Pressure controller 40 thus sets the speed of motor 4a of centrifugalpump 4 and therefore the output from pressure controller 40 at any giventime represents the actual speed of the pump 4 at that time. This pumpspeed output signal is supplied via lines 41 and 42 to an input ofpressure signal processor 13 so that the processor 13 can select theconstants B_(n) of equation (4) using equation (5). That is, processor13 can be provided with equation (5) for constants B_(n) as a functionof RPM. Cavity pressure P_(c) can then be calculated using the methoddescribed above.

A different and presently preferred method used to calculate cavitypressure, P_(c), eliminates the need to calculate Q_(in) from P_(p) andthe pressure drop caused by conduit 7. Instead, the pressure vs. flowcharacteristics of the inflow pump 4 and conduit 7 are lumped togetherand empirically determined as a single component; therefore, Q_(in) canbe determined directly from P_(s). The pressure vs. flow characteristicsof the inflow cannula assembly 9 are as described previously in equation(2):

    (2) P.sub.c =P.sub.s -A.sub.1 Q.sub.in.sup.2 -A.sub.2 Q.sub.in

Since P_(s) is measured, and A_(n) are predetermined empirically, P_(c)can be calculated, knowing Q_(in).

To calculate Q_(in), the pressure vs. flow characteristics of inflowpump 4 in communication with conduits 2 and 7 from fluid source 1 to thepoint of pressure measurement by transducer 11 were determinedempirically over a range of speeds of motor 4a. This empirical pressurevs. flow relationship was accurately fit with the following function:

    (9) P.sub.s =E.sub.0 -E.sub.1 Q.sub.in.sup.2 -E.sub.2 Q.sub.in

which can be rewritten as: ##EQU3##

As described above the functions for each B_(n) can be expressed asfunctions of RPM by:

    (11) E.sub.n =f(RPM)

and RPM can be expressed as a function of the pulse width modulationsignal, (PWM), by:

    (12) RPM=C·PWM

Although the value for C in equation (12) is the same as the value for Cin equation (6), the values for E_(n) derived from equation (11) are notthe same as the B_(n) values derived from equation (5).

Given the sensed pressure, P_(s), and the pulse width modulation signal,(PWM), body cavity pressure, P_(c), is calculated in the followingmanner:

a) Given PWM, calculate RPM from equation 12;

b) Given RPM, calculate E_(n) from equation 11;

c) Given E_(n) and P_(s), calculate Q_(in) from equation 10; and

d) Given P_(s), A_(n) and Q_(in), calculate P_(c) from equation 2.

Equations (2), (10), (11), and (12) can be solved using appropriatesoftware routines of a conventional nature.

AND gates 60a and 60b (FIG. 2) are software conditional statements, butcould be implemented in hardware, if desired. To the input of AND gate60a is applied the pump speed signal via lines 41 and 43. Unless anoverride signal is provided via line 52 at the other input of AND gate60a, the pump speed PWM signal is sent via line 61 to a variable speedpump motor 4a to raise or lower the pump speed so that it equals thepump speed represented by the pump speed signal.

A fault condition controller 50 is provided to generate an overridesignal if any of a number of system faults is detected. To the inputs50a of controller 50 are applied signals representing faults such asexcessive body cavity pressure, excessive inflow, sensor failure, etc.Controller 50 generates a fault signal in a conventional manner, whichis sent via lines 51 and 52 to the AND gate 60a and via lines 51 and 53to the AND gate 60b associated with the suction pump 18. The presence ofa fault signal at the input of gates 60a and 60b provides a pump OFFsignal as the output of gates 60a and 60b that will shut down motors 4aand 18a of centrifugal inflow pump 4 and suction pump 18, respectively.

Gates 60a, 60b, processor 13 and controllers 40, 50 are preferablyprovided by a microprocessor.

Tool 15 and tool control unit 21 are preferably provided by the PS 3500motor drive and PS 3500 EP control unit, respectively, available fromSmith & Nephew Dyonics Inc., Andover, Mass. Tool 15 will thus contain amotor, a coupler for accepting a desired surgical blade and an internalpassageway for the flow of liquid from the surgical site through theblade and tool to the conduit 16 and thence to suction pump 18. Toolcontrol unit 21 contains a power source and a controller for the toolmotor. As is known, the DYONICS PS-3500 control unit stores the rangesof tool motor speeds suitable for each of the DYONICS surgical bladesthat can be used with the DYONICS PS-3500 motor drive, and automaticallydisplays this range to the surgeon on displays 6b (FIG. 1) after theblade is inserted into the PS-3500 motor drive. The surgeon selects ablade speed within this range by operating selector 6a (FIG. 1). SeeU.S. Pat. No. 4,705,038, issued Nov. 10, 1987.

Control unit 21 generates output signals representing the blade selectedand the speed of the tool motor, the signals being sent via line 23 tolines 23a and 23b (FIG. 2), respectively, and thence to suction pumpcontroller 70. This information is processed by the suction pumpcontroller 70, which sends an output signal representing the desiredsuction pump motor speed via line 70a to AND gate 60b. If the tool motoris OFF, the suction pump motor may be OFF or run at a low speed, such asup to about 200 RPM. If the tool motor is ON, depending on the bladeselected, the suction pump motor speed will be in the range of about 500to about 4,000 RPM. The suction pump motor speed signal is sent by ANDgate 60b to pump motor 18a via line 70b, unless a fault signal has beenapplied to the input of gate 60b by line 53.

No feedback loop is provided for motor 18a. The outflow rate of liquidis solely a function of the blade used in tool 15 and whether the motor(not shown) of tool 15 is running or idle.

A manual override selector 71 is provided to enable the surgeon tooverride controller 70 to select a low, medium or high speed for motor18a for different outflow scenarios. Fault condition controller 50 willalso override controller 70 by sending a fault signal via line 53 to aninput of AND gate 60b, which will shut down pump motor 18a, as describedabove.

FIGS. 6 and 7 show an operative cannula 9a and irrigation extender 9bthat snap together to form the inflow cannula assembly 9 illustrated inFIG. 8. Operative cannula 9a has a needle portion 91 terminating indistal end 92. Bores 93a and 93b are sealed by seal 94, which whenopened by seal piercer 95 (FIGS. 7 and 8) of extender 9b, allows fluidto flow through the cannula assembly 9 into body cavity 10.

Extender 9b is provided with bore 96 that receives the proximal end 97of cannula 9a. Latch 104 snaps into groove 98 of cannula 9a to hold theinflow cannula members 9a, 9b together. Seal 94 (FIG. 6A) has opposedfaces 94a, 94b formed of an elastomer with transverse slits 94c, 94dformed therein and an outer mounting ring 94e. Seal 94 may be formedusing a removable insert between the faces 94a, 94b. Seal piercer 95passes through slits 94c, 94d to open the seal; the seal 94 beingresealed when extender 9b is uncoupled from cannula 91.

Extender 9b is provided with a rotatably mounted inlet 99 thatcommunicates with the interior bore 99a via inlet bore 99b and apertures99c. When endoscope 8 (FIG. 8) is inserted into the inflow cannulaassembly 9, inflow liquid flows through bore 99b of inlet 99 and exitsdistal end 92 via apertures 99c and the annulus 99d between the tube 91of cannula 9a and the endoscope 8.

Endoscope 8 may be secured to extender 9b by a bayonet lock (not shown)using post 99e to facilitate locking of the extender 9b to endoscope 8.Endoscope 8 is a conventional endoscope having eyepiece 8b, lighttransmitting optics 8c and light inlet 8d.

Referring to FIG. 9, a piezoresistive bridge pressure transducer 11 iscarried by connector 100. Commercially available transducers can be usedif modified to use biocompatible materials. Connector 100 is formed of afront portion 101 and rear portion 102. Front portion 101 has aninternal bore 103 for receiving the inflow inlet 99 of extender 9b,which is held in place by the spring-loaded latch 104 being insertedinto groove 98 in inflow inlet 99. Insertion of inflow inlet 99 intobore 103 will also open spring-loaded valve 101a in front unit 101. Rearunit 102 is provided with a stepped bore 104a, 104b connected byshoulder 104c. Transducer 11 is glued to the underside of unit 102 withbore 105 in unit 102 in liquid communication with bore 11a in transducer11 such that liquid flowing through conduit 7 will fill bores 105 and11a to come into direct contact with sensing diaphragm 11b of transducer11. Bores 105 and 11a are of small diameter, such as about 0.04 inches,and diaphragm 11b is hence in contact with a small column of liquid thatis at the same pressure as the inflow liquid. Transducer 11 transmits avoltage signal corresponding to the pressure, P_(s), sensed by diaphragm11b to processor 13 via line 12.

Of course, pressure transducer 11 can be located anywhere between thepump 4 and body cavity 10, with the body cavity pressure beingcalculated as described above. If the transducer 11 is located at thepump 4, then only the pressure drop between the pump 4 and cannula tip92 need be taken into account.

Alternatively, a pressure sensing tube (not shown) may be used,communicating at one end with the body cavity 10 or with conduits 7 or16 immediately outward of body cavity 10 and utilizing air as thepressure transmission medium, as is known. See, e.g., DeSatnick et al.U.S. Pat. No. 4,650,462. Other pressure sensors may also be used.

In FIG. 10, outflow cannula 14 is shown assembled to blade 15a of asurgical tool 15. Outflow cannula 14 is preferably identical to theoperative cannula 9a and has a luer taper 14a in bore 14b and a doublelead screw 14c at its proximal end 14d. Blade 15a has a complementaryluer taper 15b and double lead thread 15c so that blade 15a can besealingly fastened to the cannula 14. Tubular portion 15d opens andpasses through seal 94. Blade 15a is operatively connected by shank 15eto a tool 15 (FIG. 1). Suction applied by suction pump 18 will aspiratefluid from body cavity 10 through the tubular portion 15d into tool 15.Fluid exits tool 15 via outlet 15f (FIG. 1) into conduit 16.

FIGS. 11-15 describe a disposable pump cassette 200 containing thecentrifugal inflow pump 4 and a gear pump 18 serving to aspirate fluidfrom body cavity 10. Pumps 4 and 18 are housed in plastic housing 201having recesses formed therein during the molding of housing 201 toprovide a centrifugal pump chamber 202 (FIG. 12), and an inlet conduit203 and an outlet conduit 204 connected between the centrifugal pumpinlet 205 and outlet 206, respectively, and chamber 202. Centrifugalpump impeller 207 is received in chamber 202 and is secured to plasticshaft 207a. Impeller 207 is made of plastic and is provided with curvedvanes 208 formed during the molding of impeller 207. Impeller 207 iscemented to plastic spacer 209 and electroless nickel-plated low carbonsteel cross-member 210 (FIG. 13). The widths of the arm 210a and hub210b are empirically determined to maximize the torque imparted to theimpeller 207. Impeller assembly 207, 207a, 208, 209, 210 is spaced fromthe bottom 201a so that it may freely rotate.

Motor 4a is provided with plastic member 301, magnets 302 and ironcross-member 303 that are assembled together and are rotated by shaft300. Motor 4a is secured to housing 305, which is contained within fluidmanagement unit 5. Magnets 302 are magnetically coupled to the ironcross-member 210 such that impeller 207 will rotate at the same speed asplastic disc 301 when motor 4a is operated to rotate shaft 300 and disc301.

Suction pump 18 is a displacement pump provided by gears 211, 212.Driving gear 211 is carried by a rotatable plastic rotor 211a (FIG. 14)cemented to plastic spacer 209 and assembled to iron cross-member 210 inthe same manner as described above. Driven gear 212 is rotatably mountedin housing 200 by providing a suitable recess for receiving thegearshaft of gear 212.

Housing 201 has recesses therein formed during the molding thereof toprovide a cavity 213 for receiving the rotor assembly 211a, 209, 210, acavity 211b for receiving gear 211 and an inlet conduit 214 and outletconduit 215 connected between the suction pump inlet 216 and outlet 217,respectively, and cavity 211a. Rotor assembly 211a, 209, 210 is spacedfrom bottom 201a so that it may freely rotate. Shaft 300 of motor 18a,which is supported on member 305 within fluid management unit 5, carriesthe disc and magnet assembly 301, 302, 303, so that operation of motor18a causes rotor 211a and gear 211 to rotate, thereby rotating themeshing gear 212. Fluid is pumped out of outlet 217 by the displacementpumping action of gears 211, 212.

Housing 201 is closed by cover 200b, which is in liquid-sealingengagement with O-rings 221 and 222 (FIG. 11). Bottom 201a sealinglyengages O-ring 223 (FIG. 12).

The apparatus of the present invention may be operated as follows.Inflow cannula assembly 9 and outflow cannula 14 are inserted into thebody cavity 10, endoscope 8 is inserted into body cavity 10 via inflowcannula assembly 9 and conduit 7 is connected between fluid managementunit 5 and the inlet 99 of extender 9b. If the surgeon intends toexamine the site before inserting tool 15 into cavity 10, then outflowconduit 16 is preferably connected directly between fluid managementunit 5 and outflow cannula 14. In such a case, the appropriate speed forpump 18 is determined by manual override 71. Otherwise, outflow conduit16 is connected between fluid management unit 5 and the outlet 15f oftool 15 as shown in FIG. 1. In either case, the desired pressure in bodycavity 10 is maintained by the feedback loop described above, while theoutflow flow rate is determined independently of the pressure in bodycavity 10 by the nature of the blade in tool 15 and the speed of themotor in tool 15.

Set pressure generator 30 is then operated to select a pressure suitablefor the surgical site. For example, the selected pressure may be withinthe ranges set forth below:

    ______________________________________                                        Surgical Site                                                                              Pressure Range (mmHg)                                            ______________________________________                                        Ankle         80-150                                                          Knee         35-90                                                            Shoulder      80-150                                                          Wrist        30-80                                                            User defined <150                                                             ______________________________________                                    

FIG. 16 illustrates a presently preferred embodiment in which some ofthe elements shown in FIGS. 1 and 2 have been eliminated and/orreplaced. In particular, the suction pump 18 has been eliminated fromthe cassette 200, and the suction pump motor 18a and its associatedcontrols have likewise been eliminated. Other modifications have beenmade to the system shown in FIG. 2, as will be discussed hereinafter.Those elements from FIG. 2 that have been retained are illustrated inFIG. 16 using the same reference numerals as used in FIG. 2.

Referring to FIG. 16, the outlet of tool 15 is connected via tubing 16,20 and waste containers 401, 402 to a source of suction 403, such aswall suction, whereby liquid may be aspirated from body cavity 10.Connected between the ends of tubing 16, 20 is a length of tubing 400carried by cassette 250 (FIG. 17). A normally closed pinch valve 500(FIGS. 19A, 19B) restricts the flow of liquid flowing through tubing 16,20 by the degree to which the tubing 400 is pinched or crimped. Pinchvalve 500 may be set to provide no flow of liquid in its "closed"position or a nominal flow of liquid, as desired. Pinch valve controller404 will process the output signals in lines 23a, 23b representing theblade selected for tool 15 and the speed of the tool motor and willgenerate an output signal 405 representing the desired degree of openingof the pinch valve 500, which in turn permits the desired rate of flowof liquid through tubing 400 and tubing 16,20. Output signal 405 will beapplied to pinch valve 500 by AND gate 60b via line 70b unless a faultsignal has been applied to the input of gate 60b by line 53. Manualoverride selector 71 allows the surgeon to override the pinch valvecontroller 404 to select the desired degree of crimping of tubing 400 bypinch valve 500 to thus obtain low, medium or high outflow flow ratesthrough tubing 400 and tubing 16,20.

Suction source 403 aspirates liquid from joint 10 into waste containers401, 402 connected in series. A suitable number of containers 401, 402is provided to accommodate the predicted volume of waste liquid. Suctionsource 403 may be hospital or office wall suction or a stand alonewater-aspirator or the like.

FIG. 16 shows that the pressure transducer 257 is upstream of the bodycavity 10. As will be described hereinafter, pressure transducer 257(FIG. 18) is located at the outlet of centrifugal inflow pump 4.

The speed of motor 4a is determined by tachometer 406 and thisinformation is provided via lines 42a and 42b to motor speed controller407, which also receives the pump speed signal via line 61. Motor speedcontroller 407 compares the desired motor speed signal to the actualmotor speed signal sent by the tachometer 406 and sends a signal tomotor 4a via line 42c representing the pump speed necessary to raise orlower the calculated body cavity pressure to equal the set pressure. Themotor speed signal is also fed back by tachometer 406 to an input ofpressure signal processor 13 via lines 42a,42d.

The presently preferred embodiment of pressure signal processor 13 usedin FIG. 16 employs a simplified version of the pressure signal processordescribed earlier. First, rather than estimating the pump speed usingthe PWM signal as described above, the pump speed is monitored directlyfrom the tachometer 406 feedback signal, TAC, sent via line 42d. Second,the need to calculate the flow rate, Q_(in), from equations (4) and (8)or from equation (10) was eliminated, thus significantly reducing thetime for processor 13 to calculate the estimated body cavity pressure,P_(c). This was accomplished by using a direct relationship between theflow related pressure drop, P₁, upstream of the sensor 257 to the fluidsource 1 and the flow related pressure drop, P₂, downstream of thesensor 257 to the cannula tip 92.

Another simplification was made because it was empirically determinedthat, for a given centrifugal pump design, the coefficient B₀ inequation 4 and E₀ in equation 10 were strictly dependent on TAC and theheight of the fluid supply 1 above the pump 4, but were not dependent onflow. The remaining variables B₁, B₂, E₁, and E₂ were actually constantsthat were dependent only on the design of the tubing 2,7 and cannula 9.Since the design of the tubing, cannula and pump are fixed, so will bethe associated variables, B₁, B₂, E₁, and E₂.

The variables B₀ and E₀ described previously are dependent on the heightof fluid source 1 above pump 4, the pump speed, RPM, and the design ofthe centrifugal pump 4. Therefore, since the pump design will be fixedand the pump speed is measurable, the height of bags 1 above pump 4 mustbe either fixed or a measurable variable. Since the presently preferredembodiment fixes the height of fluid supply bag 1 at two feet above pump4 (45 mmHg), the processor 13 software contains the variable P₀ (bagoffset) preset to a constant of 45 mmHg, but the bag height offset maybe used as a variable input with appropriate change in the processorsoftware.

Given a fixed centrifugal pump design, the peak pressure that thecentrifugal pump can generate with no flow through it is a function oftachometer speed, TAC, by:

    (A) P.sub.z =f(TAC)=G.sub.1 (TAC).sup.2 +G.sub.2 (TAC).

The peak zero flow pressure that the system can generate is alsodependent on the fluid supply bag offset, P₀ ; therefore the total zeroflow pressure, P_(t), that the system can generate is defined by:

    (B) P.sub.t =P.sub.z +P.sub.o.

The pressure drop upstream of the sensor, P₁, across the tubing 2 andcentrifugal pump 4, is related to the flow through them by,

    (C) P.sub.1= P.sub.t -P.sub.s =H.sub.1 Q.sub.in.sup.2 +H.sub.2 Q.sub.in

where P_(s) is the sensed pressure, and P_(t) is the total zero flowpressure described earlier. Equation C is the same relationship asequation 9 where P_(t) =E₀, H₁ =E₁, and H₂ =E₂. The values for H₁ and H₂are dependent on the placement of the pressure sensor 257 in the outflowpath of the centrifugal pump, the geometry of the centrifugal pump, thegeometry of tubing 2 and the size of the spikes (not shown). Because theentire inflow tube set 2 and pump 4 are manufactured under tighttolerancing, it has been found that the values for H₁ and H₂ do notchange significantly from assembly to assembly. Therefore the followingpressure conversion algorithm can apply to any such pump and inflowtubeset assembly as long as the values for A₁ and A₂ in equation 1 andH₁ and H₂ in equation C are not significantly changed from setup tosetup.

The pressure drop, P₂, downstream of the sensor across the tubing 7 andcannula 9, is related to the flow through them by equation D, which isthe same as equation 1:

    (D) P.sub.2 =P.sub.s -P.sub.c =A.sub.1 Q.sub.in.sup.2 +A.sub.2 Q.sub.in

where P_(s) is the sensed pressure, and P_(c) is the pressure at thedistal end 92 of the cannula 9. The constants A₁ and A₂ are dependent onthe size of the tubing 7 and the size and design of the arthroscope 8and inflow cannula 9, and therefore care must be taken so thatvariations in design or manufacturing of the different components do notsignificantly affect the overall pressure vs. flow relationship,equation D, over the desired ranges. If changes do occur, then processor13 will be provided with software that will update the processor withthe different constants A₁ and A₂.

Although this is not the presently preferred embodiment, if one wereable to directly measure both the flow rate, Q_(in), and pressure,P_(s), then P_(c) could be estimated from equation 2 directly,

    (2) P.sub.c =P.sub.s -A.sub.1 Q.sub.in.sup.2 -A.sub.2 Q.sub.in.

Alternatively, the total pressure drop through the system, (P_(t)-P_(c)) could be used, which equals P₁ plus P₂ and can be representedas,

    (E) P.sub.t -P.sub.c =(A.sub.1 +H.sub.1)Q.sub.in.sup.2 +(A.sub.2 +H.sub.2)Q.sub.in

in which case only the flow rate, Q_(in), would be needed to estimatebody cavity pressure, P_(c), because P_(t) is known as a function ofTAC. This was not implemented due to the high cost of using both apressure sensor and a flow sensor in the system, and because a means wasfound to estimate the flow, Q_(in), from the known characteristics ofthe centrifugal pump and tubing 2,7.

The presently preferred embodiment effectively estimates the body cavitypressure, P_(c), by calculating the pressure drop downstream of thepressure sensor, P₂, from the measured pressure drop upstream of thepressure sensor, P₁. This is possible because since the flow through thebag 1--tubing 2--pump 4 and the flow through the tubing 7--cannula 9 arethe same, there must be a direct relationship between the pressure dropacross the bag 1--tubing 2--pump 4, P₁, and the pressure drop across thetubing 7--cannula 9, P₂. Since P₂ is a function of flow, Q_(in), and P₁is also a function of flow, Q_(in), then P₂ must be a function of P₁ bythe relationship

    (F) P.sub.2 =k(P.sub.1)

which can be predetermined and stored in the program memory forprocessor 13.

Pressure signal processor 13 receives a pressure signal, P_(s), frompressure transducer 257 via line 12. Pressure signal processor 13 scalespressure signal P_(s) to obtain a pressure value in units of mmHgpressure

    (G) P.sub.s (mmHg)=P.sub.s /16.

Pressure signal processor 13 also receives a tachometer signal, TAC,from tachometer 406 from line 42d. Pressure signal processor 13calculates the zero flow pump pressure, P_(z), using equation A,

    (A) P.sub.z =f(TAC)=G.sub.1 (TAC).sup.2+ G.sub.2 (TAC)

and then calculates the total zero flow pump pressure, P_(t), usingequation B, where P₀ is the known height of bag 1 above pump 4:

    (B) P.sub.t =P.sub.z +P.sub.0 =G.sub.1 (TAC).sup.2 +G.sub.2 (TAC)+P.sub.0.

By definition P₁ is the pressure drop upstream of the pressure sensor,as described by equation C,

    (C) P.sub.1 =P.sub.t -P.sub.s (mmHg).

From equation D, P₂ =P_(s) -P_(c), and from equation F, joint pressureP_(c) can be defined in terms of the calculatable pressure drop P₁ by,

    (G) P.sub.c =P.sub.s -k(P.sub.1)

which can then be sent via 32a to be represented in pressure display 5b.

Pressure signal processor 13 also communicates with pressure controller40 via lines 32 and 32b. Pressure controller 40 receives the values forP₁ from pressure signal processor 13 and calculates the total pressuredrop, P_(d), through the pump, tubing and cannula by,

    (H) P.sub.d =P.sub.1 +P.sub.2 =P.sub.1 +k(P.sub.1).

A running average of 16 calculations per second of P_(d) is maintainedto filter out turbulent noise from the motor 4a. From equation E andknowing the desired set pressure, P_(set), pressure controller 40calculates the required peak pump pressure, P¹ _(z), to overcome thetotal pressure drop, P_(d), through the system by,

    (I) P.sup.1.sub.z =P.sub.set +P.sub.d -P.sub.0

Because a centrifugal pump cannot produce negative pressures, P¹ _(z) isbounded to positive values. The TARGET tachometer speed is calculated bysolving equation B for TAC, which can be written as ##EQU4## Since P¹_(z) is known from equation I, equation J can be solved for TARGET. TheTARGET pump speed is then sent to the motor speed controller 407 vialines 41 and 61. If TARGET is more than 100, then pressure controller 40produces a TARGET signal equal to 100, to maintain a maximum pump speedof 4000 RPM, since for the motor 4a used RPM=40 TAC.

It is presently preferred to solve equations A and I by means of alookup table stored in memory listing values of P_(z) and theircorresponding TAC values.

To summarize, processor 13 and pressure controller 40 calculate theTARGET speed of motor 406a and hence the speed of pump 4 using as inputsthe motor speed, TAC, the sensed pressure, P_(s), and the set pressure,P_(set), based upon the pressure drops upstream of the sensor to thefluid supply and downstream of the sensor to the inflow cannula tip.

In the preferred embodiment described, motor 4a is a brushless,three-phase DC motor, obtained from BEI KIMCO Magnetics Division, SanMarcos, Calif., Part No. DIH 23-20-BBNB, controlled by a microprocessor.While conventional microprocessor controls can be used, it is presentlypreferred to use the brushless motor control system described in thecopending application of Kenneth W. Krause, Ser. No. Ser. No. 867,871filed concurrently herewith, and entitled Brushless Motor ControlSystem.

Cassette 250 is shown in FIG. 17 in its vertical position as viewed fromthe rear. For clarity, the front and rear covers 262, 263 (FIG. 18) havebeen omitted from FIG. 17.

Cassette 250 is made of molded plastic and houses the centrifugal inflowpump 4 disposed vertically, rather than horizontally as in cassette 200(FIGS. 11-12). Centrifugal inflow pump 4 in cassette 250 is identical tocentrifugal inflow pump 4 in cassette 200.

Within cassette 250 is pump chamber 202 and inlet and outlet conduits203, 204 connected between pump inlet 205 and pump outlet 206,respectively, and chamber 202. Pump 4 is composed of elements 207-210 asdescribed before and is driven by motor 4a and its elements 300-303 asdescribed before, except that the motor 4a is mounted horizontally, (notshown) in motor support 280 (FIG. 18). Motor 4a drives pump 4 in thecassette 250 in the same manner as described above. Rear cover 263 ofthe cassette 250 (FIG. 18) has an aperture (not shown) to allow thehorizontally mounted motor 4a to be closely adjacent the wall portion250a (FIG. 17) and hence adjacent to pump 4.

Cassette 250 is provided with a thin, flexible diaphragm 251 suitablymade of silicone rubber that closes the open end 252 (FIG. 17A) ofchamber 253. Flexible diaphragm 251 is securely held in place by clamp254 by locking the legs 255 into slots 256. Rear cover 263 (FIG. 18) hasan aperture 263a exposing the clamp 254 and diaphragm 251. As best seenin FIGS. 17 and 18, a tap hole or channel 204a in outlet conduit 204permits liquid under pressure delivered by pump 4 to enter chamber 253,thereby exerting a pressure on diaphragm 251 equal to the output staticpressure of pump 4, which is inversely related to flow through conduit204.

Cassette 250 is mounted vertically on the outside of fluid managementunit 5 by means of brackets 270, 271 (FIGS. 18, 19A, 19B) such that therear cover 263 of cassette 250 rests flush against the unit 5, alignedby pins 264. Within unit 5 is a pressure transducer 257 (FIG. 18), whosepressure-sensing element 258 is urged by spring 259 into contact withthe diaphragm 251. The relatively small hole 204a and large chamber 253filled with liquid and air tends to damp high frequency pressurevariations of liquid delivered by pump 4 due to flow turbulence. Theoutput of pressure transducer 257 is fed by wires 260 to the input ofpressure signal processor 13.

Cassette 250 (FIG. 17) also includes a length of resilient siliconetubing 400 held between connectors 400a and 400b. Tubing 16 and 20,shown in phantom lines, connects tubing 400 to the tool 15 and thesuction source 403 (via waste tanks 401, 402), respectively.

Pinch valve 500 includes an arm 501 that is reciprocated between itsnormally closed position (FIG. 19A) and its fully open position (FIG.19B), by the linear actuator motor 502, which, in turn, is controlled bythe pinch valve controller 404 and AND gate 60b, as described above.Thus, the normally closed, fully open and intermediate positions of arm501 are determined by the blade selected for tool 15 and the speed ofthe tool motor. Linear actuator motor 502 is located inside the fluidmanagement unit 5 with arm 501 extending out of the side wall of unit 5as shown. While FIGS. 19A and 19B show tubing 400 fully closed and fullyopen, as discussed above, the "closed" and "open" positions may be lessthan fully closed or fully open, as desired. Rear cover 263 is slottedto allow the arm to freely move between its open and closed positions.

Before power is initially supplied to the motor 502, arm 501 is fullyextended (FIG. 19B) to allow the cassette 250 to be inserted in positionon fluid management unit 5. As cassette 250 is lowered into place ontobrackets 270,271, tubing 400 will fit behind the finger 501a. A pair ofalignment pins 264 on fluid management unit 5 are arranged to fit intorecesses 265 in rear cover 263 when cassette 257 is in its properposition. (FIGS. 19A, 19B). After the cassette 250 is snapped in place,the tubing 16, 20 is connected. When power is supplied to the fluidmanagement unit 5, motor 502 moves arm 501 to its normally closedposition.

FIG. 20 shows a disposable operative cannula 600 having body portion601, opposed proximal and distal ends 602,603, and internal bore 604extending from end to end. The body portion 601 is provided with spacedapart, external circumferential ribs 614. Sealing members 605,606 whichare suitably made of silicone rubber, are secured to distal end 602 toseal the bore 604. Member 605 has a circular aperture 605a therein forsealingly engaging a tool, such as a powered shaver, inserted intocannula 600. Member 606 has sealing elements 606a,606b on either side ofslit 606c. Slit 606c is shown as a straight slit, but Y-shaped slitsetc. may be used.

Removal of a tool from cannula 600 or inserting a switching stickthrough cannula 600 from the distal end 603 to the proximal end 602 willcause elements 606a,606b to flex toward member 605. Member 606 istherefore spaced distally of member 605 by a distance that preventsextrusion of elements 606a,606b through aperture 605a. Accordingly,since sealing elements 606a,606b are unsupported by member 605, theymust be sufficiently thick to be stiff enough to withstand the backpressure of the liquid in cannula 600, such as about 0.075 inches thick.

If desired, member 605 can be provided with the slit and member 606 withthe circular aperture (not shown). In such a case, the sealing elements606a,606b must be spaced from member 605 to prevent extrusion ofelements 606a,606b through aperture 605a when a tool is inserted intocannula 600.

Cannula 600 includes conduit 607 for aspirating liquid from cannula 600or for supplying liquid to cannula 600. In either case, push valvemember 608 will be pushed through fitting 609 from the closed positionshown in FIG. 21 to its fully open position (not shown) in whichtransverse bore 610 is aligned with bores 611,612 in conduit 607 andfitting 609, respectively. Cannula 600 is conveniently molded fromsuitable plastics. Push valve 608 is suitably molded from siliconrubber, preferably with a hardness greater than Shore A 70. Push valve608 is color coded such that the red end 608a is showing when valve 608is closed, and the green end 608b is showing when valve 608 is open.

Cannula 600 desirably includes a circular groove 613 formed in circularbody portion 601 near the distal end 603. If the surgeon desires ashorter cannula 600, the body portion can be cut through the groove 613leaving a shorter cannula having a tapered tip.

FIG. 22 shows a diagnostic cannula 700 having a body 701 and a rotatablymounted inlet 702 that communicates with the internal bore 703 via inletbore 702a and apertures 702b. Diagnostic inflow cannula 700 is differentfrom inflow cannula assembly 9 described earlier. Without extender 9b,cannula 700 permits the surgeon to reach deeper into the surgical site,which is helpful specifically for shoulder arthroscopy. Althoughdifferent in application, cannula 700 and cannula assembly 9 haveidentical pressure vs. flow characteristics, as is required for pressuresignal processor 13.

For portal interchangability, cannula 600 is available with an innerdiameter suitable for inserting inflow cannula 700 through cannula 600.For example, an endoscope may be first inserted and locked into cannula700 and remain assembled throughout the surgical procedure. If thesurgeon has two operative cannulas 600 in place in the body, one can beused for a tool 15 and the other for the inflow cannula 700. The inflowcannula 700 and endoscope assembly is inserted through sealing members605,606 until the distal end 703 of body 701 abuts the seal member 605.Push valve member 608 is moved to the closed position. Inflow liquidflows through bore 702a and exits tube 704 through apertures 702b andthe annulus between the endoscope and the tube 705. The fluid andoperative debris within the surgical site is removed either through thesuction adapter on the surgical tool, or can be removed through conduit607 of operative cannula 600 being used with the surgical tool, in whichcase push valve member 608 is moved to the open position.

FIG. 23 shows a modified distal tip for the operative cannula of FIG.20. Thus, the cannula 600 of FIG. 20 has a tapered distal end 603, withinternal bore 604 extending through body 601 from the proximal end 605to and through the distal end 603. In FIG. 23, however, cannula 600terminates in the distal end 603a which has a tapered obturator tip 603bclosing the internal bore 604. Liquid exits internal bore 604 viaapertures 603c spaced circumferentially about body 601 just upstream ofobturator tip 603b. With this modified distal end 603a, the cannula 600acts as its own obturator and can be directly advanced through the bodywithout the use of a separate obturator.

FIG. 24 shows a further cannula 800 having a body portion 801 withexternal ribs 802. An internal bore 803 extends from the proximal end804 through the body portion 801, with tapered obturator tip 805 closingthe bore 803. Liquid exits internal bore 803 via apertures 805. Cannula800 is made of suitable plastic and can also be advanced into the bodywithout the need for a separate obturator.

We claim:
 1. Apparatus for supplying liquid under pressure to a bodycavity and establishing a predetermined body cavity pressure during anendoscopic procedure, which comprises:a) a centrifugal inflow pump forsupplying liquid at a pressure suitable for said predetermined bodycavity pressure: b) means for generating a body cavity pressure signalrepresenting the pressure in said body cavity comprising pressure sensormeans in communication with liquid under pressure delivered by saidcentrifugal inflow pump at a location upstream of said body cavity andpressure signal processing means for calculating the value of thepressure in said body cavity from said sensed pressure and forgenerating a calculated body cavity pressure signal as said body cavitypressure signal; c) conduit means for communicating said centrifugalinflow pump with said body cavity to deliver liquid under pressure tosaid body cavity; d) control means responsive to said body cavitypressure signal for increasing or decreasing the pressure of said liquiddelivered by said centrifugal inflow pump to thereby maintain said bodycavity pressure at said predetermined pressure; e) said centrifugalinflow pump, when supplying said liquid at said suitable pressure,having an outflow flow rate that is inherently inversely responsive tochanges in said body cavity pressure; f) a liquid outflow cannula forinsertion into said body cavity; and g) a source of suctioncommunicating with said liquid outflow cannula for aspirating liquidfrom said body cavity via said liquid outflow cannula.
 2. Apparatusaccording to claim 1, wherein said source of suction is a suction pump.3. Apparatus according to claim 1, wherein tubing means connects saidsource of suction to said liquid outflow cannula and means is providedselectively to crimp said tubing and thus selectively restrict flow ofliquid aspirated from said body cavity.
 4. Apparatus according to claim1, wherein said pressure sensor means comprises a pressure transducermeans and an operative pressure-sensing diaphragm, and said pressuresignal generating means includes channel means for directing a column ofliquid from liquid flowing through said conduit means to said diaphragm.5. Apparatus according to claim 4, wherein said pressure sensor meanscomprises a chamber having an open end, a thin, flexible diaphragmclosing said open end, channel means for directing liquid delivered fromsaid liquid supply means to said chamber, and pressure transducer meansin operative contact with said flexible diaphragm.
 6. Apparatusaccording to claim 1, wherein said location of said pressure sensormeans is adjacent the outlet of said centrifugal inflow pump. 7.Apparatus according to claim 6, wherein said pressure sensor meanscomprises a chamber having an open end, a thin, flexible diaphragmclosing said open end, channel means for directing liquid delivered fromsaid pump outlet to said chamber, and pressure transducer means inoperative contact with said flexible diaphragm.